1. Field of the Invention
The present invention relates to a throttle control apparatus for an internal combustion engine adapted to drive a throttle valve placed in an intake passage of the internal combustion engine by using a driving device to cope with icing of the throttle valve.
2. Description of Related Art
As this type of apparatus, there is conventionally known for example a throttle control apparatus disclosed in Japanese examined patent publication No. 4(1992)-4452. This throttle control apparatus is arranged to control a throttle valve for preventing icing or freezing thereof in a cold environment. The icing of the throttle valve indicates the phenomenon in which vapor and fuel contained in intake air freeze to form ice on or around the throttle valve when the intake air is low in temperature and high in humidity during engine warm-up condition, thus causing blockage of an intake passage. In some cases, further, the icing may cause the engine to stop. For avoiding such troubles, the above throttle control apparatus comprises operating condition detection means for detecting an engine operating condition, icing detection means for detecting whether the throttle valve is in a frozen or iced state, throttle valve opening/closing means for electrically opening and closing the throttle valve, and a control circuit for controlling the throttle valve opening/closing means. In this apparatus, an accelerator opening sensor is used as the operating condition detection means and a temperature sensor and a humidity sensor are used as the icing detection means. The throttle valve opening/closing means includes a DC servomotor and its drive circuit. The control circuit is arranged to execute “icing elimination control” which comprises driving the DC servomotor and others according to an engine operating condition based on a signal from the accelerator opening sensor, detecting whether the throttle valve is frozen based on a signal from the temperature sensor, the humidity sensor, and others, and, when detects the icing (the frozen state), controlling the DC servomotor and other components to swing the throttle valve at a predetermined cycle in such a small opening range as to be around the opening degree suitable for the current operating condition without causing no variation in an engine rotational speed.
However, in the throttle control apparatus disclosed in the '452 publication, the control circuit neither detects whether the throttle valve is frozen nor executes the icing elimination control unless a specific environmental condition depending on the accelerator opening degree, temperature, and humidity is satisfied. This apparatus therefore could not cope with icing if occurred under any environmental conditions other than the specific environmental condition. Further, the specific environmental condition depending on the accelerator opening degree, temperature, and humidity is merely determined by estimating a condition that icing is likely to occur and also anticipating the occurrence of icing. Accordingly, even when the control circuit determines whether icing has occurred based on the specific environmental condition, there is a possibility that no icing has occurred actually. In other words, it appears that this throttle control apparatus prospectively detects (estimates) whether the icing has occurred based on the specific environmental condition. Thus, this apparatus would be low in accuracy of icing detection. Further, this throttle control apparatus is arranged to merely swing the throttle valve around a certain target opening degree in order to eliminate icing, which could not produce sufficient torque to the throttle valve. It is consequently concerned that this operation of the throttle valve could not generate sufficient icing elimination power to remove solid or hard frozen ice.